Drilling the back gives you alittle more room in the tank. I'm assuming that's why you want to drill? Just make sure to cap the bottom holes really well to prevent future leakage. Also drilling your own lets you control the flow of the tank better then previously drilled holes. Although most the time that's not a huge issue anyways.
You gonna paint the background black? I've seen some nice jags on the blue background looks really good. But I've always been partial to black.
As for your sump,what type of stand is it? Because like the others said 55 is great! Except you won't be able to remove it. If it ever leaks or cracks ECT or there is something you want to change. If your stand is like most wooden stands with two center braces a 4" tank gets tricky.
If you can fit the 4" that's awesome. I personally use a bag filter with submerged media. But I've used trickle towers before with filter floss on the top too they work just as well. I actually just got a slightly bigger one myself. So now I have my heater and a large sponge filter in mine as well. So it keep items in my tank to a minimum.
But when you need a sponge for that hospital tank you have one.
As for a pump, I currently use a Jecod/Jebao DCT Marine Controllable Water Pump. It's quite, powerful, and best of all cheap. It has a nifty feed feature. And soft start. I've been running one in both my 125 and 150 for about 6 months now with no hiccups.
Anyways long rant over. Good luck with the tank man. Can't wait to see that guy become a beast.